FlyVirgin744
Topic Author
Posts: 1282
Joined: Tue Jul 27, 1999 8:35 am

### Calculus Help!

Alright all you smart people:

Regarding related rates,

Assume x and y are both differentiable functions of t and find the required values of dy/dt and dx/dt .

Equation: xy = 4

Find: dy/dt when x = 8
Given: dx/dt = 10

and

Find: dx/dt when x = 1
Given: dy/dt = -6

Matt
Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.

PanAm747
Posts: 4713
Joined: Mon Mar 01, 2004 4:46 am

### RE: Calculus Help!

you have to use the multiplication rule:

xy'+yx'=4

I think, it's been a while.
Pan Am:The World's Most Experienced Airline - P(oor) S(ailor's) A(irline): San Diego's Hometown Airline-Catch Our Smile!

FlyVirgin744
Topic Author
Posts: 1282
Joined: Tue Jul 27, 1999 8:35 am

### RE: Calculus Help!

Panam, that rule does come into play but it gets quite a bit more complicated. Thank you for your response though.
Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.

pgh234
Posts: 753
Joined: Tue Jul 10, 2001 12:48 pm

### RE: Calculus Help!

OMG, that is SOOOOOO creepy. I am doing the EXACT SAME PROBLEM for my AP Calculus homework as I am reading your post. Do you happen to have the same book as me also? It is a red Calculus book with "Larson-Hostetler-Edwards" Second edition on the bottom of the cover and the problem you are refering to is on Page 178, problem #3a in section 2.7?

anyways, to solve:

xy = 4 dx/dt=10 and find dy/dt when X=8

(x dy/dt + y dx/dt) = 0 (Multiplication Rule as stated above)
dy/dt = -(y/x)dx/dt
dy/dt = - ((1/2)/8) * 10 (Found Y value by solving equation for X=8)
dy/dt = -5/8

Good Luck in Calculus, cuz so far, it sucks!
pgh234

FlyVirgin744
Topic Author
Posts: 1282
Joined: Tue Jul 27, 1999 8:35 am

### RE: Calculus Help!

Thank you so much! Seems so easy now

I am actually using the 6th edition of your book! Its brand new, could have some better examples though.

Well thanx again and good luck to you also in your class. And oh yes, it definetely does suck.
Sometimes I go about in pity for myself and all the while a great wind carries me across the sky.

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